March 7, 2011Q: Your 5/25/10 post on the John Lennon clip-on sunglasses is interesting but impossible to find. I've worn P3's for years. Good enough to storm the beach at Normandy, still good enough for me. But P4's? Can't find them and I've asked some old optometrists and they've never heard of them either. Google doesn't turn up any clues either. Any suggestions?--Scott

A: We've asked our glasses expert for further clarification. "Even people in the industry don't use the terms 'P3' or 'P4' correctly, or at all," he says. "Find 3 people who use the words, and you'll get 3 different explanations, ranging from 'It's a military code word for the frame style,' to 'It's the relationship between the a and b dimensions.'

"Our understanding is that P3 refers to '3 points' -- the P3 shape is like a rounded, upside down triangle ... it has 3 points. Similarly, a P4 has 4 points and is usually a trapezoidal shape. If you look carefully at the photo of Lennon with sunglass clips, you can make out that he has P3 frames underneath and a P4 sunglass clip over them."

To attain the same effect, we recommend you purchase these vintage Polaroid aviator clip-ons. That way, your brain can imagine the world living as one all it wants, but your face will be packing the fire-power of two branches of our Armed Forces.